What if it's the only thing in the universe and it's massless? How would you know if it was moving or not without anything to compare it to? Couldn't it be said to be stationary? If a photon moves by definition, then can it be said that a photon can't exist without something else existing to compare it's speed to?

What if it's the only thing in the universe and it's massless? How would you know if it was moving or not without anything to compare it to?

If it is the only thing in the universe, then why would you care if it moves or not? There's nothing else to determine what it is doing. The "you" in your scenario already contradicts your premise of it being the only thing in the universe.

There's nothing else to determine what it is doing. The "you" in your scenario already contradicts your premise of it being the only thing in the universe.

Zz.

Sure, but I'm pretending I'm an ethereal goddish figure in this case. Do I have to be in that universe also in order to conceive of the idea?

Motion is only defined relative to other objects. So if something is the only object it cannot have motion. But a photon by definition must move. In a universe with only one photon, the photon must move at c, but yet also cannot move. It's a paradox. So how can it exist in this situation? Wouldn't you have to redefine movement or redefine a photon to get it to exist by itself (or add another particle to measure against)? You don't have to do that with a particle that doesn't have a set speed. Doesn't this suggest that somethng very odd is going on with light - as if we didn't already know?

A photon can be at rest with respect to some other object? I would say no.

Well, you have a point -- anything without mass necessarily moves at c. I don't actually know the pedantic answer for whether two photons travelling in the same direction can be said to be at rest with one another.

Sure, but I'm pretending I'm an ethereal goddish figure in this case. Do I have to be in that universe also in order to conceive of the idea?

Motion is only defined relative to other objects. So if something is the only object it cannot have motion. But a photon by definition must move. In a universe with only one photon, the photon must move at c, but yet also cannot move. It's a paradox. So how can it exist in this situation? Wouldn't you have to redefine movement or redefine a photon to get it to exist by itself (or add another particle to measure against)? You don't have to do that with a particle that doesn't have a set speed. Doesn't this suggest that somethng very odd is going on with light - as if we didn't already know?

Then I can also be a goddish figure (after all, if you can, so can I) and tell you that it makes no sense to define something that only exists in that universe. After all, if we're making things up and setting up our own rules, my rules should work as well.

This then shows that this made-up scenario is meaningless and proves nothing.

A photon can be at rest with respect to some other object? I would say no.

What about two photons traveling parallel to each other (previously mentioned)? I think this demands an answer. We can't just dismiss this as an irrelevant oddity. Something has to give here. We have to postulate an odd solution. Perhaps they CAN be stationary relative to something else. Perhaps there is no "other" photon and all photons are one so the question is moot. Perhaps we have a mistaken notion of what motion is all about. Perhaps photons can't travel parallel to each other. Perhaps photons don't even move in the first place. Perhaps the photons ARE moving apart at c, but the space between them is shrinking at c. Perhaps they are moving apart at c in another dimension. Perhaps there are actually no photons. I don't know. I can come up with lots of other bizarre things like this that are probably way off the mark - but maybe not. It's time to think outside the box on this one. Let's not avoid the issue.

This then shows that this made-up scenario is meaningless and proves nothing.

I'll accept that it proves nothing, but it's certainly not meaningless to ask about. It's an important thought experiment. At the very least it makes you think and that's a positive thing. Are you saying nothing can be gained from imagining bizarre situations?

Staff: Mentor

What about two photons traveling parallel to each other (previously mentioned)? I think this demands an answer. We can't just dismiss this as an irrelevant oddity. Something has to give here. We have to postulate an odd solution. Perhaps they CAN be stationary relative to something else. Perhaps there is no "other" photon and all photons are one so the question is moot. Perhaps we have a mistaken notion of what motion is all about. Perhaps photons can't travel parallel to each other. Perhaps photons don't even move in the first place. Perhaps the photons ARE moving apart at c, but the space between them is shrinking at c. Perhaps they are moving apart at c in another dimension. Perhaps there are actually no photons. I don't know. I can come up with lots of other bizarre things like this that are probably way off the mark - but maybe not. It's time to think outside the box on this one. Let's not avoid the issue.

I know it's fun to make up stuff and ask off-the-wall questions, but give us a break with the melodrama.

Two photons moving side by side will of course both move with respect to any local reference frame at speed c. As far as what the speed of one photon is with respect to the other, that question has no meaningful answer as one cannot define a reference frame for a photon.

Note that if the photons are moving towards (or away from) each other, there's no problem in saying that they are closing (or separating) at the rate of twice the speed of light--again, as measured with respect to some reference frame.

Sure, but I'm pretending I'm an ethereal goddish figure in this case. Do I have to be in that universe also in order to conceive of the idea?

Motion is only defined relative to other objects. So if something is the only object it cannot have motion. But a photon by definition must move. In a universe with only one photon, the photon must move at c, but yet also cannot move. It's a paradox. So how can it exist in this situation? Wouldn't you have to redefine movement or redefine a photon to get it to exist by itself (or add another particle to measure against)? You don't have to do that with a particle that doesn't have a set speed. Doesn't this suggest that somethng very odd is going on with light - as if we didn't already know?

Is it possible to define motion, then as being relative to some arbitrary coordinate system? Nothing else would technically have to exist in the physical sense for such a quantity to be calculated in this case, I don't think.

I'll accept that it proves nothing, but it's certainly not meaningless to ask about. It's an important thought experiment. At the very least it makes you think and that's a positive thing. Are you saying nothing can be gained from imagining bizarre situations?

The most important aspect about a thought experiment is that it must be grounded in something that is a possibility under an ideal situation! Yours is neither grounded, nor a possibility. It serves no purpose.

The most important aspect about a thought experiment is that it must be grounded in something that is a possibility under an ideal situation! Yours is neither grounded, nor a possibility. It serves no purpose.

Zz.

I'm unsure as to how someone can define a thought experiment's purpose other than in a subjective manner. One could argue that the entire purpose of a thought experiment is just... to think... no?

I would respectfully disagree there. It makes you think. It makes you consider things you might not have considered before. It makes you ask yourself why it's not possible. Plus, in order for you to say it's not possible, you would need to either explain how the laws of physics forbid it or explain how it's logically self-contradictory. That explanation would be illuminating.

As far as what the speed of one photon is with respect to the other, that question has no meaningful answer as one cannot define a reference frame for a photon.

Why not? Isn't the photon at rest with regard to a parallel one? Why isn't that an inertial reference frame? Why do you have to define it relative to a non-photon for it to make sense? I'm sure I just don't understand relativity well enough. I'm just trying to learn.

How do you "explain" something when the premise doesn't allow you to make any "frame" for you to based any explanation on?

You are forgetting that the ability to "explain" something requres an a priori agreement of the rules that everyone agrees on. Here, it is physics. When you set up your scenario, have you considered what physics have you disallowed? There's nothing to detect this photon in a solitary universe. Your question itself is "logically self-contradictory". How come that realization isn't "illuminating" in itself?

Two photons moving side by side will of course both move with respect to any local reference frame at speed c. As far as what the speed of one photon is with respect to the other, that question has no meaningful answer as one cannot define a reference frame for a photon.

Exactly.

Note that if the photons are moving towards (or away from) each other, there's no problem in saying that they are closing (or separating) at the rate of twice the speed of light--again, as measured with respect to some reference frame.

? What does it mean? For relativistic speeds, you can't use V = v1 + v2 (or V = v1 - v2);
you should use V = (v1 + v2)/(1 + v1v2/c^2) and, however, you can't use even that one in the case of two photons.

Staff: Mentor

Note that if the photons are moving towards (or away from) each other, there's no problem in saying that they are closing (or separating) at the rate of twice the speed of light--again, as measured with respect to some reference frame.

? What does it mean? For relativistic speeds, you can't use V = v1 + v2 (or V = v1 - v2);
you should use V = (v1 + v2)/(1 + v1v2/c^2) and, however, you can't use even that one in the case of two photons.

Reread what I wrote. Example: In the lab frame I observe photon A moving to the right towards photon B which moves to the left. At some time (according to the lab frame), each photon is one light year away from me, thus 2 light years apart. When do they reach me? In one year. Thus they covered a distance of 2 light years in one year.

I generally refer to this as the third-party separation rate, since it is the separation rate of two objects with respect to a third (not each other). Not to be confused with the speed of one photon with respect to the other.